1. Field of Use
The invention relates to a multiband antenna structure suitable for mobile stations in particular.
2. Description of Related Art
The development in mobile stations is leading towards apparatuses functioning in a plurality of frequency bands. At first, mobile phones operated each in one frequency band only. Then, models functioning in two frequency bands, such as GSM900 and GSM1800 systems, were introduced into the market. The introduction of WCDMA (wideband code division multiple access) technology and extension of the GSM (global system for mobile communications) technology into the frequency band earlier used by the NMT450 system result in the manufacture of models functioning in even more bands. This means additional requirements accordingly on the antennas of the mobile communication devices in question.
From the prior art it is known several antenna structures functioning in at least two frequency bands. These structures usually comprise two parts arranged to resonate at different frequency bands. The parts may be separate helix elements, a helix and a whip, branches of the radiating plane of a planar inverted F type antenna (PIFA), or meander and planar elements on a surface of a printed circuit board, for instance. An arrangement is also known in which the fundamental frequency of an individual antenna element falls into the band used by a radio system, and a harmonic of the fundamental frequency falls into the band used by another radio system. An example of this is shown in FIG. 1. The figure shows a helix-type antenna element 110, connecting piece 120, and an antenna feed conductor 130. The element 110 is at its lower end attached to the connecting piece 120 which in turn can be screwed into the body or shell structure of the apparatus. In the element 110, the pitch, or the distance between two successive turns, measured in the direction of the axis of the element, gets smaller approaching the upper end of the element. For example, a pitch p1 is smaller than a pitch p2 at a lower point in the element. The feed conductor 130 is galvanically connected to the lower end of the helix element 110 and extends downwards through an axial hole in the connecting piece 120. The connecting piece may be conductive, but isolated from the helix, whereby it further serves as a shield for the end portion of the feed conductor. FIG. 1 further shows in dashed line also the outline of the protective hood of the antenna.
All antenna elements resonate at a certain fundamental frequency and also at harmonic frequencies of the fundamental frequency. The fundamental frequency refers here to the frequency at which the electrical length of the antenna element equals a quarter of the wavelength. If the pitch in a helix antenna is uniform throughout, the harmonics are multiples of the fundamental frequency. If there are different pitches, the relation between the harmonics changes. By suitably selecting the number of turns and the pitches, among other things, it is possible to have such values for the resonance frequencies that the structure is applicable in the frequency bands of at least two radio systems. Usually the fundamental frequency and its third harmonic are arranged at the desired locations on the frequency axis. The third harmonic means the third resonance frequency when the fundamental frequency is counted as the xe2x80x9cfirst harmonicxe2x80x9d.
A common drawback in arrangements according to FIG. 1 and in comparable arrangements in respect to multiband use is that there are only two or at the most three frequency bands that can be used. It is true that by adding elements one can have as many bands as one wants, but then the structure with its matchings becomes bulky and complicated.
An object of the invention is to reduce said drawbacks associated with the prior art. A structure according to the invention is characterized by that which is specified in the independent claim 1. Some advantageous embodiments of the invention are specified in the other claims.
The basic idea of the invention is as follows: The antenna structure has as its radiating elements not only the helix but also the joining piece that attaches the helix to the apparatus. The helix is designed such that the distance between its conductor turns varies. The electrical length of the joining piece is increased by means of a conductive projection, for example, which remains within the covering of the apparatus. By suitably dimensioning the parts at least five of the resonances that the helix and the joining piece have together, are arranged at useful locations on the frequency scale.
An advantage of the invention is that it facilitates an antenna suitable for mobile stations, which antenna has more bands than prior-art antennas. Another advantage of the invention is that the production costs of the structure according to the invention are relatively small.